Known then as "Marine A", in 2013 Blackman was convicted of murder at Bulford Court Martial over the fatal shooting of an injured Taliban insurgent in 2011 and jailed for life.

Mr Blackman left prison shortly before 12.20am on Friday. Credit: PA

The appeal court accepted the grounds of diminished responsibility as he was suffering from a mental illness, known as adjustment disorder.

It was argued that during his tour of duty in Helmand during 2011 Sergeant Blackman and his men had been overstretched and under resourced.

"I will be eternally grateful to Claire and I cannot put into words how wonderful she is," he told the Daily Mail.

"She is a wife in a million. Other inmates often said how lucky I was to have her fighting so hard for me. I don't think there is anybody who has witnessed the effort she has gone to who will doubt how she feels about me, and that's beyond words really. You just can't imagine anyone cares for you that much," he said.

His life sentence was reduced to seven years, meaning he was eligible for release on licence after serving half the term.

Blackman said he was "very conscious" his sentence was not complete and he will have to adhere to the conditions of his parole - but he can now enjoy the "little things" such as walking, eating and sleeping when he chooses.

He described the feeling of waking to "glorious birdsong" as a "massive contrast" to life at the Category C prison.

Mrs Blackman fought for her husband's conviction to be reviewed and he was granted an appeal in February.